Electric cable



P. V. HUNTER ELECTRIC CABLE Sept. 21, 1937.

Filed Sept. 22, 1934 ig. I.

R p. m m V M d d d m m :w W W m 6M V., /.N 0.4M? F M/ [mow pi m3 Em2702/ mica/6 Patented Sept. 21, 1937 UNITED STATES PATENT OFFICEELECTRIC CABLE Application September 22, 1934, Serial No. 745,080 InGreat Britain October 10, 1933 2 Claims.

This invention relates to the manufacture of high tension electriccables of the kind in which layers of paper are applied to the conductorby a wrapping process to form a laminated body of dielectric that issubsequently impregnated with a liquid or semi-liquid insulatingmaterial. The invention deals particularly with the method ofimpregnating the dielectric body surrounding the conductor.

In accordance with the invention the process of impregnating thedielectric is carried out in two stages. In the first stage of theprocess the previously dried laminated dielectric body of papersurrounding the conductor is impregnated with a mobile insulating oil.After this stage surplusinsulating oil unabsorbed by the paper, that isto say, the oil that is present in the space between the paper lappingsand in the interstices in the conductor, is removed from the insulatedconductor under conditions that do not result in removal by evaporationof the oil absorbed by the paper. In the second stage, which followsthis draining operation, the dielectric body of paper is impregnatedwith a relatively viscous insulating oil or compound. The first stage ofimpregnation may be carried out in the usual manner by first drying thedielectric with the assistance of heat and evacuation and thenimpregnating with a hot mobile oil under pressure. Heat and evacuationalso precede the second stage of impregnation so as to insure completeremoval of all free mobile oil and to facilitate the introduction of thehot impregnated compound in the second stage.

The oil' for the first stage of impregnation has a sufllciently lowviscosity and is sufiiciently fluid at the temperature used duringimpregnation to impregnate the fibres of the paper, that is to say, toenter the cells and canals within the structure of these fibres as wellas to occupy the smallest 40 spaces between the fibres. The subsequenttreatment of the insulated conductor does not remove the oil fromtheinterior of the fibres or from the spaces between them. Thetemperature'at which the first impregnation takes place will bedetermined partly by the degree of evacuation preierred. The temperatureat which the draining operation is carried out will also depend to acertain extent upon the oil used in the first stage and will of coursebe well below the boiling point of that oil under conditions of pressureprevailing.

in the first stage. The impregnating temperatures suitable for anyimpregnating medium selected will either be apparent to the cablemanufacturer or may be readily ascertained by him. As an example ofsuitable oil for use in the first stage of the process may be mentionedtransformer oil, that is the oil generally employed in the tanks ofstatic electric transformers.

The oil or compound used for the second impregnation is suflicientlystiff at normal temperatures to prevent running[ It has been foundpreferable to employ an oil which though more viscous in character thanthe oil employed in the first stage, is an oil of the same base. Thisoil may, if desired, contain in admixture therewith rosin of a suitablegrade.

The process may be carried out in the following manner. The paperinsulated conductor is first dried by any of the known methods andinserted in a vacuum-tight impregnating vessel. The vessel is thenevacuated and the first impregnating medium is then drawn in at asuitably high temperature. If transformer oil is used a suitabletemperature is from 245 to 250 F. The impregnating fluid and theinsulated conductor are maintained at this temperature until the fibresof the paper have become thoroughly impregnated with the fluid. Theimpregnation may be assisted by subjecting the fluid in the vessel to asuitable pressure and/or by evacuating the insulated conductor from theinterior. After the impregnation of the fibres is complete, the surplusfluid which lies on the surfaces of the papers and, where the conductoris a stranded conductor, in the interstices of conductors, is removed,either by draining the mass of fluid from the vessel or by removing theinsulated conductor from the vessel and allowing it to drain underatmospheric pressure. After the draining process has been carried out,the insulated conductor is again placed under vacuum in the impregnatingvessel and the normally viscous fluid is drawn in at a temperature thatis sufliciently high to render the fluid sufliciently mobile to enableit to fill the interstices in the body of insulation and in theconductor. Where transformer oil is used in the first stage and moreviscous oil of the same base is used in the second stage, a suitabletemperature for carrying out the second stage of impregnation is thetemperature that was stated to be suitable for the first stage ofimpregnation, namely, a temperature of 245 to 250 F. As in the firststage, the impregnation in the second stage may be facilitated bysubjecting the fluid to pressure and/or placing the interior of theinsulated conductor under vacuum. After cooling, the insulated conductoris withdrawn and furnished with an impervious sheath of lead or othermaterial.

The accompanying drawing shows diagrammatically and by way of exampleonly, a lead sheathed single core electric cable of which the laminatedpaper insulation thereof has been impregnated in accordance with thepresent invention. In this drawing,

Figure l is a side elevation of the cable from which portions have beenremoved to expose the construction of the interior of the cable andFigure 2 is a fragmentary cross-section on an enlarged scale of thecable shown in Figure 1.

By referring to the drawing it will be seen that the cable conductor Iconsists of a plurality of wires stranded together and that theinsulation consists of a plurality of layers 2 of paper, each layerconsisting of a helical lapping of paper tape. The insulated conductoris enclosed in a lead sheath 3. The fibres'in the paper layers 2 arecompletely impregnated with a mobile insulating liquid and the spacesbetween the successive lappings of paper and the interstices in theconductor are filled with insulating material 4 which is suflicientlyviscous at normal temperaturesto prevent it flowing along the cable. Theouter surface of the insulation is also coated with the viscousmaterial. In this way the insulating fluid within the paper is sealed inplace by the surrounding viscous material 3.

It will be understood that the illustration of the structure in Figure 2is diagrammatic, since, for clearness, it has been necessary to departfrom the appropriate relation between the dimensions of the parts. Eachpaper layer 2 will have a thickness of a few thousandths of an inchwhile each layer 4 of impregnating mate'- rial between the paper layerswill have a thickness of a fraction of a thousandth of an inch.

The result of the improved method of manufacture is an impregnated paperinsulated conductor or cable in which the interior structure of thepaper remains substantially completely impregtion, is prevented. The oilretained after this first stage is substantially all bound to theinternal parts of the paper structure.

It will be apparent that the mechanical construction of the cable needin no way be afiected by the use of the improved impregnating processwhich may be employed in connection with the manufacture of single ormulti-core type paper insulated cable.

I claim as my invention:--

1. The method of insulating the conductor of I an electric cable, whichcomprises enveloping the conductor in a laminated body of paper, dryingthe said laminated body, impregnating the dried laminated body with aninsulating liquid that is mobile at normal atmospheric temperatures, re-1 moving from the paper enveloped conductor with the assistance of heatand evacuation and under conditions that do not result in evaporation ofthe said mobile insulating liquid absorbed by the paper; surplus mobileinsulating liquid unabsorbed by the paper, and subsequently impregnatingthe laminated body with a relatively viscouse insulating liquid.

2. The method of insulating the conductor of an electric cable whichcomprises enveloping the conductor in a laminated body of paper, dryingthe said laminated body with the assistance of heat and evacuation,impregnating at an elevated temperature the said dried body with aninsulating liquid that is mobile at normal atmospheric temperatures,removing in the liquid state from the, paper enveloped conductor, withthe assistance of heat and evacuation, surplus insulating liquidunabsorbed by the paper, and subsequently impregnating the laminatedpaper body, under conditions that do not result in evaporation of themobile liquid absorbed by the paper, with a relatively viscousinsulating liquid that at normal temperatures is highly viscous.

. PHILIP VASSAR HUNTER.

